1. Field of the Invention
The present invention generally relates to a laser diode with enhanced optical and current characteristics and method for fabricating the same.
2. Discussion of Related Art
The most widely used semiconductor laser diodes in the 635 nm, 650 nm and 670 nm bandwidth of visible ray are typically index wave-guide type laser diodes with an InGaP/AlGaInP heterostructure.
The epitaxial structure for fabrication of such a semiconductor laser diode is prepared on a substrate which is normally misoriented by some degrees (e.g., 9 or 15 degrees) in order to have a shorter wavelength for prevention of a superlattice structure naturally occurring in the crystal growth and for enhancement of optical recording density.
Accordingly, the ridge formed by a wet etching in preparation of device becomes asymmetric due to the effect of the substrate.
This leads to a decrease in the parallel far-field angle out of the characteristics of laser diode and deteriorates the mode stabilization.
Further, the instability of wet etching affects the larges-caled productivity and the product yield.
Such a conventional laser diode causing the above problems will be described in more detail with reference to the attached drawings.
FIG. 1 is a cross-sectional view showing an index guided AlGaInP laser structure which is widely used as an SBR (Selectively Buried Ridge) wave-guide laser diode fabricated by a wet etching.
In order to maintain a uniform thickness xe2x80x9ctxe2x80x9d, a laser structure has been developed that has an etch stop layer which is several tens of angstroms in thickness, as shown in FIG. 2.
It is actually necessary to maintain the thickness xe2x80x9ctxe2x80x9d, which is the most important factor that affects the mode characteristics, optical characteristics and current characteristics of laser.
Such an SBR structure as illustrated in FIG. 2 has a great advantage in the simplicity of fabrication process but basically involves a structural problem.
That is, the ridge prepared by a wet etching using an insulating layer as a mask has a great difference between top and bottom ridge widths WT and WB, and is of an asymmetric shape as seen from the angles a and b due to the substrate characteristic.
The top ridge width WT relates to the threshold current Ith out of the current characteristics, while the bottom ridge width has a close relation with the parallel far-field angle out of the optical characteristics.
To regulate top and bottom ridge widths WT and WB with respect to a given size of the insulating layer in the SBR structure, it is necessary to obtain a ridge structure with enhanced both threshold current and parallel far-field angle for the prevention of trade off caused between the threshold current and the parallel far-field angle.
That is, an increase in the bottom ridge width WB intended to increase the parallel far-field angle reduces the top ridge width WT below a specified level, which results in a rapid increase in the threshold current, thereby limiting the regulation of bottom ridge width WB.
Inversely, an increase in WT intended to reduce the threshold current causes WB to increase, decreasing the parallel far-field angle.
To overcome the problem, many studies have been tried in fabrication of a vertical ridge structure using a reactive ion etching, as illustrated in FIG. 3.
Reactive ion etching is an anisotropic etching technique by which it is easer to regulate an etching depth than by using wet etching, and accordingly, it is more widely used to realize an asymmetric vertical ridge in an epitaxial structure grown on a misoriented wafer.
As shown in FIG. 3, the difference between WT and WB is substantially small enough to satisfy the above-mentioned characteristics relative to that caused in the ridge structure obtained by using a wet etching as shown in FIG. 2.
However, an etch stop layer is not used in the method with reactive ion etching, and accordingly, there are some problems that it depends upon the uniformity of the reactive ion etching and the capability to regulate the etching depth, compared to the step in FIG. 2.
Furthermore, a reactive ion etching causes damages on the surface, which becomes troublesome in the growth of n-GaAs current blocking layer.
As described above, a conventional laser diode involves some problems as follows:
A laser diode fabricated by a wet etching has an asymmetric ridge structure, which decreases the parallel far-field angle and deteriorates the stability of mode.
A laser diode fabricated by a reactive ion etching is hard to etch uniformly and to regulate etching depth.
Accordingly, the present invention is directed to a laser diode and method for fabricating the same that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a laser diode with enhanced optical and current characteristics and method for fabricating the same by using both wet etching and reactive ion etching techniques.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a laser diode includes: a first conductivity type clad layer formed on a first conductivity type substrate; an active layer formed on the first conductivity type clad layer; a second conductivity type first clad layer formed on the active layer; an etch stop layer formed on the second conductivity type first clad layer; a second conductivity type second clad layer vertically formed on the lateral sides thereof with a specified width in a defined region on the etch stop layer; a second conductivity type InGaP layer formed on the second conductivity type second clad layer and having a width identical to that of the second conductivity type second clad layer; a current blocking layer formed on both lateral sides of the second conductivity type second clad layer; a second conductivity type contact layer formed on the current blocking layer and the second conductivity type InGaP layer; and electrodes formed beneath the substrate and on the second conductivity type contact layer, respectively.
In another aspect of the present invention, a method of fabricating a laser diode includes the steps of: sequentially forming a first conductivity type clad layer, an active layer, a second conductivity type first clad layer, an etch stop layer, a second conductivity type second clad layer, a second conductivity type InGaP layer, and a second conductivity type GaAs layer, on a first conductivity type substrate; forming an insulating layer on the second conductivity type GaAs layer and patterning it, exposing a defined region of the second conductivity type GaAs layer; performing a reactive ion etching using the patterned insulating layer as a mask, etching the second conductivity type GaAs layer, the second conductivity type InGaP layer, and the second conductivity type second clad layer to a specified depth and remaining part of the second conductivity type second clad layer; forming a photoresist on the whole surface including the insulating layer and patterning it, exposing the residual second conductivity type second clad layer; performing a wet etching using the patterned photoresist as a mask to etch the second conductivity type second clad layer, exposing the etch stop layer and etching the residual photoresist and insulating layer; forming a current blocking layer on the exposed etch stop layer, and a second conductivity type contact layer on the whole surface including the current blocking layer; and forming electrodes on the second conductivity type contact layer and beneath the substrate, respectively.
In further another aspect of the present invention, a method of fabricating a laser diode includes the steps of: sequentially forming a first conductivity type clad layer, an active layer, a second conductivity type first clad layer, an etch stop layer, a second conductivity type second clad layer, a second conductivity type InGaP layer, and a second conductivity type GaAs layer, on a first conductivity type substrate; forming a first insulating layer on the second conductivity type GaAs layer and patterning it, exposing a defined region of the second conductivity type GaAs layer; performing a reactive ion etching using the patterned first insulating layer as a mask, etching the second conductivity type GaAs layer, the second conductivity type InGaP layer, and the second conductivity type second clad layer to a specified depth and remaining part of the second conductivity type second clad layer; sequentially forming a second insulating layer and a photoresist on the whole surface including the first insulating layer, and patterning the photoresist, exposing the second insulating layer on the residual second conductivity type second clad layer; performing a wet etching using the patterned photoresist as a mask to etch the second insulating layer and the second conductivity type second clad layer, exposing the etch stop layer and etching the residual photoresist and first and second insulating layers; forming a current blocking layer on the exposed etch stop layer, and a second conductivity type contact layer on the whole surface including the current blocking layer; and forming electrodes on the second conductivity type contact layer and beneath the substrate, respectively.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.